Surface micromachining of a microelectromechanical systems (MEMS) structure fundamentally involves the deposition and patterning of at least two types of layers on top of a substrate. The first type of layer includes one or more structural layers in which the final MEMS structure is formed. The second type of layer includes sacrificial layers which support or encase the MEMS structure during formation of the MEMS and which is subsequently dissolved away resulting in a released MEMS structure.
The sacrificial layers are usually dissolved by immersing the devices in a wet chemical etchant for accomplishing the release of the MEMS structure. The sacrificial layers are typically disposed between the bottom substrate and the top MEMS structure layers. The wet chemical etching process etches the sacrificial layers between the MEMS structure layers and the substrate thereby releasing the MEMS structure from the substrate. Stiction occurs while the MEMS structure is immersed in a liquid etchant. Stiction also occurs while the MEMS structure is being slowly removed from the etchant or during rinsing with deionized water. Surface tension forces generated during the wet release process tend to cause stiction whereby a smooth underside surface of the released MEMS structure sticks to an even smoother substrate surface below the MEMS structure resulting in the MEMS structure being stuck to and in contact with the substrate. Such contact between the MEMS structure and substrate disadvantageously renders the MEMS structure immobile preventing proper actuation, desired flexure, or release of the MEMS structure from the substrate. The MEMS structure must then be mechanically pried up from the surface of the substrate substantially increasing the likelihood of damage to the MEMS structure and substrate.
The removal of the sacrificial layer with vapor phase etching is desirable in that it replaces the whole sequence of etching, rinsing steps, and elaborate drying procedure, not to mention the fact that no meniscus is formed during the releasing procedure. The removal of a sacrificial oxide with hydrofluoric acid (HF) vapor has been tested by researchers for years to release polysilicon structures. However, in reality, condensation of water on the etching surface renders the vapor process not so dry and the results have not been so satisfactory. The condensation cannot be easily avoided since water molecules are produced on the surface as a result of chemical reaction for oxide etching. The best result reported has been obtained by heating the wafer during vapor HF etching and preventing excessive water condensation.
The HF vapor etch of the silicon dioxide sacrificial layer is easy and convenient. However, the problem of removing water from the HF vapor has been solved by providing an anhydrous HF and CH.sub.3 OH vapor mixture to release the MEMS structure in a completely dry fashion without stiction. This system disadvantageously requires the use of more complicated equipment such as an exhaust system. These and other disadvantages are solved or reduced using the invention.